1. Field of Invention
This invention relates to hollow fibre filters and more particularly to shells or casings which house the fibres and to the headers through which fluids pass to and from the shell.
2. Discussion of the Background
In this specification, the term "hollow fibre" refers to fibres of a tubular shape having a continuous passageway (or lumen) disposed substantially along the axial centre line of the fibre. The term "membrane" refers to porous or microporous material which may, inter alia, be in the shape of a hollow fibre.
Hollow fibre filters consist of a bundle of hollow, porous, polymeric fibres which can be arranged in the shell or casing in either a tube-in-shell or candle-in-shell configuration.
Tube-in-shell filters which are widely used for ultrafiltration and microfiltration consist of a number of hollow porous fibres aligned side by side as a bundle and which are secured at each end by being cast in a resin, care being taken to keep the lumens of the fibres open. The bundle thus formed is either permanently bonded at each end to an outer shell, which may be made of plastic material, or it is fitted or moulded with sealing means for insertion into a reuseable, usually metal, shell.
Candle-in-shell filters are similar except that the fibre bundle is attached to the cartridge shell at one end, and at the other the ends of the fibres are each sealed but left free of one another. Alternatively, the fibres in the bundle are each looped so that both ends of each fibre are sealed in the resin casting.
The assembly of the fibre bundle and shell forms what is called a filter cartridge. At each end of the cartridge there is a header through which fluids pass to and from the cartridge. The feed to be filtered may be applied to either he inside or the outside of the fibres with the filtrate being withdrawn from the other side of the fibres.
Cartridges and headers are often bonded or welded to one another to form an integral structure. Sometimes the whole fibre plus shell plus header assembly is called a cartridge, but in this specification the term "cartridge" applies to the assembly of fibre bundle and shell without the headers and the term "filter" applies to the cartridge plus headers.
A plurality such filters may be connected in parallel or in series and are usually coupled by threaded screw fittings to piping from a feed pump and to piping leading to a filtrate collection apparatus. The term "bank", "bank of filters", or "filter bank" is applied to such an assembly of filters. The piping assemblies which deliver feedstock to and collect filtrate and concentrate from a plurality of filters, are each called a manifold.
Existing designs have many disadvantages. For example, screw fittings are expensive and take up space. Furthermore, in many applications (such as shipboard or portable use) the maximum possible area of filter membrane must be contained in the smallest possible volume. Filters with protruding fittings do not use space economically. In addition, portable applications require light weight construction using a minimum of materials.
Another disadvantage of existing designs is that equipment incorporating lightweight cartridges and headers made of polymeric plastic materials is not transportable in an assembled or partially assembled form. Because piping and screw fittings support a cantilevered assembly, they can snap if transported on a truck in rough terrain or if delivered by helicopter or parachute.
There is a need for compact transportable equipment for mobile or military use. There is also a need that such equipment be at least partially assembled during transport and that it be easy to complete the assembly for rapid use in the field.
A disadvantage of metal shells and headers is that they are expensive, and, for economy, require that the bundle of hollow fibre membranes be replaceable within the metal shell. This is achieved with a series of O-rings at each end of the bundle which must then be inserted with considerable mechanical force into the metal tube and the metal tube reassembled into or onto the header. Fibre bundle inspection and replacement is, as a consequence, difficult.
Furthermore, it is desirable to have a range of filter dimensions made available for different applications Varying feedstocks to be filtered contain different amounts of impurities and for economy, those with few impurities should be filtered at high flux rates. Long cartridges containing fine fibres are not able to provide a high velocity of drawoff of filtrate because of the hydraulic pressure drop of high volumetric flow rates in the narrow lumens of the fibres and hence short cartridges are required. Conversely, dirty feeds require longer cartridges where the lower flux rates present no problems of lumen pressure drop. With metal shells and headers, variable cartridge and filter dimensions are expensive to implement and service.
Another problem with prior art designs arises because different types and batches of fibres have different quality in terms of initial defects or service failure rates per unit of fibre surface area. If cartridges can be manufactured cheaply enough then economy, convenience and utility can be optimised by varying the number of fibres per cartridge.
Thus, higher defect-rate fibres could be manufactured into a cartridge with fewer fibres so that the chance, and hence the penalties, of a failed cartridge would be less. Because of the limitations imposed by practically obtainable minimum defect rates, optimal cartridge diameters for industrial porous hollow fibre microfilters are usually 70 to 80 mm containing 1 to 2 square meters of membrane. These figures will, of course, increase as fibres become more reliable and cheaper, and be constrained only by practical limitations of feedstock penetration to all parts of the hollow fibre membrane bundle during operation.
As soon as one fibre breaks or develops a fault, integral plastic cartridges and headers must be replaced. Moreover, repair of damaged fibres is not economic.
Cartridges are tested for failure by means of a bubble pressure test. When water occupies all of the pores in the membrane, a certain pressure, known as the bubble point of the membrane, has to be exceeded to overcome the interfacial tension of the water in the pores. In the bubble pressure test, air is forced back into the lumens of the wetted fibre. Failed fibres allow air to pass through the fibre walls at a pressure lower than the bubble point of the membrane. The opacity of prior art industrial cartridges and headers does not allow visual detection of a failed cartridge, and each cartridge must be individually tested after first being disassembled from the filter bank. Individual small medical cartridges have usually been made transparent, at greater cost, to allow bubble testing.
Hitherto, feed has usually been pumped into the shell as a radial jet at right angles to the direction of flow within the shell. With radial introduction of feed, the fibres at the inlet end on the opposite side from the inlet receive little feed. This is because the feed stream impacts against the fibres and is diverted down them, rather than swirling round them. Similarly, in the absence of evenly distributed withdrawal of feed, dirt accumulates at the outlet end against the side of the fibre bundle remote from the outlet. A considerable amount of useful filter surface is by-passed, which is inefficient.
U.S. Patent Specification Nos. 4,565,630, 4,578,190, 4,639,353, 4,617,161, 4,568,456, 4,632,756, 4,414,113, 4,390,575, European Patent Specification Nos. 186,293, 163,900, Japanese Patent Specification Nos. 61-031,164, 60-261,507, 60-261,506, 60-061,006, 59-115,702, 58-041,830, 58-143,805, 57-159-502, 57-150,402 and United Kingdom Patent Specification No. 2,090,546 all disclose slight variations on the general principle of filter design in which a plurality of mostly hollow fibre membranes are disposed inside a shell with the ends of the fibre lumens sealed from the outside of the membranes. The shell has a feed inlet as well as a filtrate outlet and usually has a feed outlet or return, all of which are in the form of a spigot or port. None of these previous designs provides an efficient means of connecting a plurality of filters.
U.S. Pat. Nos. 4,346,006, 4,400,276, 4,497,104, 4,308,654, Japanese Patent Nos. 65-024,004, 61-157,308, 61-057,206, 59-115,702, 59-130,503, 51-093,788, 51-103,083, 56-141,801, 56-037,002, 55-157,304, 58-109,104, European Patent No. 183,256 and Russian Patent No. 1,158,211 all disclose slight variations on methods of manufacturing hollow fibre membrane cartridges and filters which have the general form of parallel or substantially parallel hollow fibre membranes sealingly enclosed in a shell to form a cartridge, with a header or shell entry and exit ports for feedstock, filtrate and concentrate, however, none of these inventions makes use of the headers as an efficient means of connecting more than one filter in a bank.
A review of header and cartridge or filter designs is given in French Patent No. 2,267,138 which discloses a hollow fibre membrane filter in which the bundle of fibres is enclosed by an elastic, tightly wrapped sheath. There is a common factor in the design of most prior art hollow fibre membrane filters, being that of disposing a bundle of mostly parallel hollow fibre membranes inside a usually cylindrical shell Sometimes the shell has inlet ports for feedstock or filtrate, however, a header will always provide a sealing means so that the ends of the lumens are separate from the outside of the fibres. The prior art does not, however, disclose an efficient means of connecting a plurality of cartridges.